1. Field of the Invention
The present invention relates to a scanning system and more specifically to a scanning system wherein a field of view is temporarily held so as to permit time integration of an image in an imaging plane.
2. Discussion of Related Art
In conventional continuous scanning imaging apparatuses such as Infrared Search and Track (IRST) systems, imaging optics continuously scan through a Field Of Regard (FOR) focusing an image of a more limited Field of View (FOV) upon a detector array. For instance, imaging objects have been mounted to a gimbal capable of rotating 360 degrees to image a field of regard of 360.degree..times.18.degree., for example. As the gimbal rotates, the imaging optics sequentially images fields of view within a field of regard. In order to perform a search and track function, for instance, a sensor must provide a rapid gimbal scan. Rapid gimbal motion (e.g., 240 degrees/sec. minimum) is only practical if the motion is smooth and continuous. The image impinging upon the detector array, however, is moving with the movement of the gimbal. Without compensation, most detectors would provide only a blurred image. This is true because detectors take some incremental amount of time to generate an electronic signal sufficient to represent the image with a certain resolution (e.g., convert enough photons into electrical signals to provide a sufficiently large image signal). The traditional solution is to provide an linear array or a focal plane array with a Time Delay and Integrate (TDI) function.
In these linear or focal plane arrays, the image moves from detector segment to adjacent detector segment as the field of regard is scanned. The resulting charges generated in the respective detector segments are stored in correspondence to each individual detector segment and shifted to the next storage location in synchronization with the image movement such that the converted electromagnetic energy from a single Line of Sight (LOS) in the field of regard is integrated as the image scans across the linear array. By this mechanism, enough electromagnetic energy is converted into electrical energy to provide a sufficient image signal.
However, TDI linear arrays are inefficient. Their inefficiency is due to the clocking time it takes for the charge corresponding to one detector segment to be transferred to an adjacent storage location and for the associated switching electronics. In other words, the total integration time is diminished by the amount of time it takes for the charge to be transferred from one storage location to the adjacent storage location and to the storage location after that. Integration time is key to any imaging system insofar as the longer the integration time is, the greater the sensitive the system.
Staring arrays, which are arrays that do not have a timed delay integrate function, are becoming increasingly important in the imaging field. A staring focal plane array may collect and integrate the signal from a scene for much longer periods of time (1 to 1,000 milliseconds) compared to a TDI device, but a sensor line of sight must be held constant and accurately stabilized to realize the gain in the sensitivity. The problem is, of course, that a staring array requires that it be pointed in a given direction for a given integration time. In a scanning system without some additional mechanism, a staring array would be inappropriate insofar as there would be little if any integration time for each field of view the staring array was focused upon. The image would tend to be blurred if some compensation mechanism were not in place.
The traditional solution for providing an integration time for a staring array in a scanning system is to move the gimbal to a field of view, stop the gimbal to permit the array to stare at the object space for an integration time, and thereafter re-initiate movement of the gimbal to the next field of view. A problem with this solution is that the gimbal tends to be somewhat massive relative to the system. It takes a relatively significant amount of power and time to start the gimbal, stop it for a given field of view and then recommence motion to the next field of view due to the gimbal's inertia. The inertia of this type of system renders the system slow and undesirable.
Whereas the power and settling time required to perform a cyclic start/stop motion with the gimbal is too large to yield a practical system, search and track systems have been limited to linear or TDI focal plane arrays. The integration time of a linear or TDI array is short, on the order of 10 to 100 microseconds. The sensitivity of the scanner limits the range in which the targets may be detected.